Mercury is an undesirable constituent of a considerable number of fluid streams, and consequently a considerable number of methods have been devised to selectively remove the mercury. In the main the mercury impurity is in the form of elemental mercury, but in a few instances mercury compounds, including organic mercury compounds, are of concern. In the case of elemental mercury the purification processes are largely adsorption procedures, and in these the most common type of adsorbent is an activated carbon having supported thereon a mercury reactive material such as potassium triodide, sulfur, sulfuric acid, chlorine, silver, copper or various salts of silver or copper. Other supports for the mercury reactive materials include silicas, aluminas, silica-aluminas and zeolitic aluminosilicates. Ion-exchange resins, particularly the strongly basic anion-exchange types which have been reacted with a polysulfide, have also been reported. See U.S. Pat. No. 4,591,490 (Horton) in this latter regard. The disclosures of U.S. Pat. No. 4,500,327 (Nishino) and U.S. Pat. No. 4,196,173 (de Jong et al) are pertinent to the use of activated carbon supports.
Perhaps the two greatest problems involved in removing mercury from fluid streams are (a) achieving a sufficient reduction in the mercury concentration of the feed stream being treated, and (b) avoiding the reentry of the recovered mercury into some other environmental medium. Although permissible levels of mercury impurity vary considerably, depending upon the ultimate intended use of the purified product, for purified natural gas, mercury concentrations greater than about 0.01 microgram per normal cubic meter (.mu.g/nm.sup.3) is considered undesirable, particularly in those instances in which the natural gas is to be liquefied by cryogenic processing. In the cases where mercury is removed from process streams by use of non-regenerable adsorbents, very large adsorption beds are required. This is because sufficient adsorbent must be present not only for the long term equilibrium capacity, but also enough adsorbent to contain the mass transfer (reaction) zone. In the case where the mercury removal is done by regenerative means, less adsorbent is required since only the adsorbent for containing the mass transfer zone is required. If regenerable, the regeneration media requirements are not only large but result in a large mercury-laden bed effluent which must itself be disposed of in an environmentally safe manner. A means has now been devised to combine the favorable aspects of both regenerable and non-regenerable process systems. Such a combination allows for (a) attaining the lowest possible mercury levels in the process streams, (b) making full utilization of the non-regenerative mercury removal adsorbent, and (c) disposing of the mercury in an environmentally safe manner.